1. Field of the Invention
The present invention relates to flowing fluid systems, both closed and open systems, in which the present invention prevents the fluid temperature from exceeding a predetermined set temperature at a point in the system, by cooling the flow in a controlled manner when required, to bring the fluid temperature down to the predetermined set-point temperature. Thus, overcooling is prevented. The present invention makes use of a thermostatic mixing valve in a completely new way to achieve this. One of the applications of the present invention is in the area of renewable energy, specifically solar thermal systems for water heating and space heating. Other applications include, but are not limited to, control of engine oil and coolant temperature, of transmission oil temperature, of hydraulic fluid temperature, of cutting oil temperature, and of storage tank fluid temperature.
2. Description of the Prior Art
Many solar thermal heating applications in non-drain-back systems suffer from overheating problems, including the loss-of-load problem and the over-supply problem. Both involve a mismatch in which the heat supply from the solar collector or collectors is greater than the heat load.
Many situations can cause a loss-of-load problem. A classic loss-of load solar-hot-water problem typically arises when a family goes on vacation in the summer without putting the solar system into vacation mode. With the storage tank at temperature, full of hot water, and no calls for hot water, the system can overheat because the solar collectors continue to add heat to the system which does not need it. Boiling and over-pressurization of the fluid in the solar collector usually follow, with accompanying damage to the system or to the fluid itself in the case of antifreeze solutions. This is an example of the loss-of-load problem.
Some solar thermal system designers opt to degrade the solar-hot-water-heating system performance to provide almost all of the hot water in summer and about half of the hot water needed in winter to avoid overheating in summer. They choose to under-size the system to avoid overheating on the hottest summer days when the solar collectors can be producing at their highest heat levels. If designers were to design a system with more solar collector area for more hot water in winter, the system could produce an over-supply of heat at times in the summer, thus potentially leading to boiling and over-pressurization of the fluid and accompanying damage as described above. This is an example of the over-supply problem.
Others have attempted to solve these problems in different ways. US Patent Number 20100059047 describes an automated over-temperature protection system that uses a pressure vessel near the outlet of the solar collector. Fluid is forced out of the solar collector and into a pressure vessel when the fluid in the solar collector boils. This protection system fails to prevent boiling before it starts. The allowed boiling can damage the system or fluid in the system.
U.S. Pat. No. 7,823,582 describes an automated solar collector temperature controller which opens dampers to the air space of the flat plat solar collector. This protection system works only for flat plate solar collectors, and compromises the thermal integrity of the solar collector with mechanical dampers which wear and eventually fail to close completely or properly.
U.S. Pat. No. 7,913,684 describes an automated protection system to remove vapor from a solar collector and indirectly cool it should fluid boiling occur in the solar collector in a pressurized solar thermal heating system. This system only addresses a loss of circulation, only works for a pressurized system, and by itself fails to prevent boiling. The patent adds dampers to the flat plate collector in the same fashion as the patent above. This damper system works only for flat plate solar collectors, and compromises the thermal integrity of the solar collector with mechanical dampers which wear and eventually fail to close completely or properly.
U.S. Pat. No. 4,102,325 describes an automated solar collector temperature control system which uses a thermosyphon, a valve and additional tubing integrated into the solar collector. This system is complex and expensive, and depends on small density differences in fluids to drive fluid flow and the cooling rate which can vary with the angle of piping and other factors, and hence can be unreliable.
Some solar thermal heating systems use heat dumps to shed excess heat. Typically, a heat dump can be a hot tub, a swimming pool, a slab of concrete with embedded hydronic tubing, a liquid-to-air heat dissipater, or other heat-dissipating device. Customary practice is to place a temperature-driven valve downstream of the outlet of the solar collectors, and to divert some or all of the flow through the heat dump. This method can overcool the fluid as there is no temperature feedback where the diverted flow returns to the system. This wastes energy, and can result in longer times to bring the storage tank up to temperature.
Some solar thermal heating systems use multiple sensors, electrically operated valves, electronic control systems, and heat dumps to limit fluid temperature. These systems are generally complex, expensive and difficult to service and to diagnose when troubles arise.
Some solar thermal heating systems use periodic heat dumping by hot water discharge to bring the temperature of storage tanks back down to within operating range. These systems risk allowing tank fluid temperatures to get too high, and waste water by discharging hot water down the drain and injecting cold water. Such systems are risky and wasteful of energy and water.
Solar thermal systems retrofit installations are infrequently done because of the expense and complexity of the installation. Much of the complexity and expense come from the lack of an available heat dump or the difficulty and expense of piping to a heat dump. When done, such retrofit installations typically are undersized to prevent problems such as boiling. Use of renewable energy in solar thermal systems is hampered by the complexity and expense of installation and is underutilized by undersizing.
Thermostatically controlled valves, both mixing valves and diverting valves, have been used in industrial applications for many years for fluid temperature control. For example, they are used in diesel-engine-based electric generators to control coolant temperature to and from the engine and lubricating oil temperature to and from the engine. These valves typically combine coolant pumped from the engine with coolant from an external heat dump, usually outside the building housing the engine. These valve arrangements do not provide in-line cooling, and require an external source of cooling.
Mixing valves are used in boiler-type heating systems for various purposes, including reducing the fluid temperature from the boiler going into a hydronic radiant floor. This system uses the return from the radiant floor as the source of colder fluid. Such a cold return is not available in a solar thermal system and many other fluid systems.
Mixing valves are also used on domestic hot water systems to reduce the risk of scalding should the water heater produce water hot enough to burn the skin. This system uses the cold water source to reduce the water temperature. Such a cold source cannot be used in a solar thermal system and many other fluid systems because the systems are closed.
Oil coolers for engines sometimes have a diverter valve where oil exits the engine and then heads to a heat dissipater circuit or back into the engine. This valve sends oil back into the engine when the valve inlet temperature is below a set value, but diverts the flow into a heat dissipater circuit when the valve inlet temperature is above the set value, before returning to the engine. This system cools the oil when it becomes too hot, but does not control the amount of cooling as flow is simply directed into a heat dissipater without regard as to the fluid temperature exiting the heat dissipater. This can result in overcooling the engine, especially in very cold climates.
Hydraulic systems need to operate within a small range of viscosity for proper operation and to avoid cutting short component life. This translates into maintaining the appropriate fluid temperature as viscosity is temperature dependent. Current systems simply have the operator watch for anomalous operation or watch temperature sensor gauges. When an over-temperature issue arises and is detected, it is usually too late, with the result being that some hydraulic component malfunctions or fails. Current systems fail to prevent or mitigate the over-temperature condition.
Cutting oils work best when they are at or below a predetermined temperature. Current cooling systems for machines that use these oils fail to limit temperature or maintain a constant oil temperature.
In addition, current fluid systems without electronic controls don't adjust to changing conditions such as ambient temperature, heat transfer rate from the heat dissipater, flow rate change from pump degradation, flow path blockage, or fluid temperature change. Adding expensive electronic controls adds to the complexity of fluid systems and extra, unnecessary expense.